1. Field of the Invention
This invention relates to steam turbines and, more particularly, to a solid particle magnetic deflection system for removing or diverting metallic oxide particles entrained in and transported by a steam flow path enroute to a steam turbine, thereby to eliminate or substantially reduce erosion damage caused by solid particle impact within the steam turbine.
2. State of the Relevant Art
Damage to steam turbines due to solid particle erosion (SPE) is a matter of growing concern. During start-up of a steam turbine plant, particularly from a cold or shut-down condition, solid particles are exfoliated, i.e. spalled, from the surface of the boiler tubes within a steam generator which supplies the high pressure steam for driving the turbine. The particles become entrained in the steam flow and are transported through interconnecting piping to the steam turbine, passing through the associated stop and control valves enroute to the high pressure turbine inlet nozzles. The erosive damage to these parts can be extensive. Further, SPE damage results in reduced efficiency, and thus lost power generation, adding to the cost for repair and replacement of eroded components.
The exfoliation, entrainment and transport of the solid particles may be a continuous phenomenon, as the boiler tubes and steam piping of a system accumulate operating time at elevated temperatures. However, the primary occurrence of exfoliation appears to be at the time of start-up, after a steam turbine plant has been brought off-line and allowed to cool, during which time a metallic oxide scale forms on the inner surfaces of the boiler tubes. It is believed and generally accepted in the industry that the amount of oxide scale buildup in the interconnecting piping is minimal, at least compared with that in the boiler tubes. As the boiler is fired, the difference in thermal characteristics between the oxide and the parent material of the boiler tube results in separation of the oxide from the tube, i.e., exfoliation. A similar situation exists at the inlet to intermediate and/or low pressure turbines, as a result of metallic oxides emanating from first (and second) reheater tube walls.
The magnitude of the SPE problem and the concentrated effort in the industry to find practical solutions for it was the main topic of a symposium conducted by the Electric Power Research Institute (EPRI) held in Chattanooga, Tenn., Nov. 13-15, 1985. The proceedings of the EPRI meeting, under the general heading "Solid-Particle Erosion of Utility Steam Turbines: 1985 Workshop", included publications by Sumner, W. J. et al. "Reducing Solid Particle Erosion Damage in Large Steam Turbines," and by Miller, V. R. D. "Maintenance Strategies and Specific Design Changes to Ontario Hydros Lambton TGS Coal-Fired Generating Units to Mitigate the Effects of Solid Particle Erosion," among others. The Sumner et al. article reports that based on recent data at that time, the cost estimates for SPE damage are as great as $150 million per year.
Proposals have been made, and EPRI has funded research programs, for combating the SPE problem, directed, variously: to particulate reduction through chemical cleaning, chromate treatment and chromizing of boilers, to particulate removal through steam/air blowdown and through use of inertial separators; and to steam turbine armoring through plasma spray, diffusion bondings, and steam turbine blade and nozzle redesign. Operational schemes have also been proposed for reducing SPE damage, such as full-arc admission (taking into account the continuity equation of flow dynamics), which serves to reduce nozzle passage velocities and thereby the erosive effects of the impacting oxide particles. These schemes, however, have not satisfactorily solved the SPE problem, as evidenced by continued EPRI research programs.
There thus exists a continuing need for reducing the damage caused by SPE and particularly the erosion caused by solid particle impacting of steam turbine components, such solid particles being entrained in the steam flow and transported through interconnecting piping to the turbine and its internal components.